We all tend to be in a more slumped position with rounded shoulders and an extended head, so working on bringing your shoulders down and back, tucking the chin in and having an upright posture will actually help alleviate a lot of stress when you're sitting. If you have a desk job, you tend to get into this position a lot, so scooting yourself back in a chair, bringing your shoulders back, creating a little arch in your low back and then raising your computer screen up to be at eye level is going to help promote this posture.
The other big thing is anytime you feel yourself getting stuck down in a bad posture, you want to bring yourself up. You want to create a new habit of new posture and the best way to do that is correcting it! Some people even set a timer to remind themselves to sit up, bring the shoulders down and back and then create a slight chin tuck so that they are in an upright position. That's going to help take a lot of stress off the upper back and your neck muscles.
A lot of people tend to get headaches because those muscles are tight from being extended back!